Gene targeting by means of homologous recombination between homologous exogenous DNA and endogenous chromosomal sequences has proven to be an extremely valuable way to create deletions, insertions, design mutations, correct gene mutations, introduce transgenes, or make other genetic modifications in mice. Current methods involve using standard targeting vectors, with regions of homology to endogenous DNA typically totaling less than 10-20 kb, to introduce the desired genetic modification into mouse embryonic stem (ES) cells, followed by the injection of the altered ES cells into mouse embryos to transmit these engineered genetic modifications into the mouse germline (Smithies et al., Nature, 317:230-234, 1985; Thomas et al., Cell, 51:503-512, 1987; Koller et al., Proc Natl Acad Sci USA, 86:8927-8931, 1989; Kuhn et al., Science, 254:707-710, 1991; Thomas et al., Nature, 346:847-850, 1990; Schwartzberg et al., Science, 246:799-803, 1989; Doetschman et al., Nature, 330:576-578, 1987; Thomson et al., Cell, 5:313-321, 1989; DeChiara et al., Nature, 345:78-80, 1990; U.S. Pat. No. 5,789,215, issued Aug. 4, 1998 in the name of GenPharm International) In these current methods, detecting the rare ES cells in which the standard targeting vectors have correctly targeted and modified the desired endogenous gene(s) or chromosomal locus (loci) requires sequence information outside of the homologous targeting sequences contained within the targeting vector. Assays for successful targeting involve standard Southern blotting or long PCR (Cheng, et al., Nature, 369:684-5, 1994; Foord and Rose, PCR Methods Appl, 3:S149-61, 1994; Ponce and Micol, Nucleic Acids Res, 20:623, 1992; U.S. Pat. No. 5,436,149 issued to Takara Shuzo Co., Ltd.) from sequences outside the targeting vector and spanning an entire homology arm (see Definitions); thus, because of size considerations that limit these methods, the size of the homology arms are restricted to less than 10-20 kb in total (Joyner, The Practical Approach Series, 293, 1999).
The ability to utilize targeting vectors with homology arms larger than those used in current methods would be extremely valuable. For example, such targeting vectors could be more rapidly and conveniently generated from available libraries containing large genomic inserts (e.g. BAC or PAC libraries) than targeting vectors made using current technologies, in which such genomic inserts have to be extensively characterized and trimmed prior to use. In addition, larger modifications as well as modifications spanning larger genomic regions could be more conveniently generated and in fewer steps than using current technologies. Furthermore, the use of long regions of homology could increase the targeting frequency of “hard to target” loci in eukaryotic cells, since the targeting of homologous recombination in eukaryotic cells appears to be related to the total homology contained within the targeting vector (Deng and Capecchi, Mol Cell Biol, 12:3365-71, 1992). In addition, the increased targeting frequency obtained using long homology arms could diminish any potential benefit that can be derived from using isogenic DNA in these targeting vectors.
The problem of engineering precise modifications into very large genomic fragments, such as those cloned in BAC libraries, has largely been solved through the use of homologous recombination in bacteria (Zhang, et al., Nat Genet, 20:123-8, 1998; Yang, et al., Nat Biotechnol, 15:859-65, 1997; Angrand, et al., Nucleic Acids Res, 27:e16, 1999; Muyrers, et al., Nucleic Acids Res, 27:1555-7, 1999; Narayanan, et al., Gene Ther, 6:442-7, 1999), allowing for the construction of vectors containing large regions of homology to eukaryotic endogenous genes or chromosomal loci. However, once made, these vectors have not been generally useful for modifying endogenous genes or chromosomal loci via homologous recombination because of the difficulty in detecting rare correct targeting events when homology arms are larger than 10-20 kb (Joyner, The Practical Approach Series, 293, 1999). Consequently, vectors generated using bacterial homologous recombination from BAC genomic fragments must still be extensively trimmed prior to use as targeting vectors (Hill et al., Genomics, 64:111-3, 2000). Therefore, there is still a need for a rapid and convenient methodology that makes possible the use of targeting vectors containing large regions of homology so as to modify endogenous genes or chromosomal loci in eukaryotic cells.
In accordance with the present invention, Applicants provide novel methods that enable the use of targeting vectors containing large regions of homology so as to modify endogenous genes or chromosomal loci in eukaryotic cells via homologous recombination. Such methods overcome the above-described limitations of current technologies. In addition, the skilled artisan will readily recognize that the methods of the invention are easily adapted for use with any genomic DNA of any eukaryotic organism including, but not limited to, animals such as mouse, rat, other rodent, or human, as well as plants such as soy, corn and wheat.